The invention relates to a shutter control means with auto strobo for a camera which incorporates an electrical shutter.
An auto strobo unit is a flash generator having an automatic light controlling capability, and comprises a flash discharge tube which is energized for illumination upon closure of the X contacts of the camera and in which the flash illumination is automatically interrupted by an illumination control circuit when a proper exposure level is reached. When the auto strobo unit is used in combination with the camera, the illumination control circuit of the flash illumination circuit within the auto strobo unit performs an arithmetic operation using aperture values preset in the camera, and interrupts the illumination of the flash discharge tube by means of an illumination stop signal when the calculated value reaches a given level or a proper exposure level. If the full illumination from the auto strobo unit is insufficient to reach the given value, the resulting photograph will be under-exposed. If the camera used is only capable of establishing an exposure period manually, a synchronized photographing will be achieved at a particular exposure period, usually 1/60 second, resulting in the closure of the shutter after 1/60 second even if the photographing is completed under an under-exposure condition. However, with an automatic exposure camera incorporating an electrical shutter which includes an automatic exposure control circuit of the type which does not employ a storage circuit, for example, an automatic exposure control circuit of direct photometry type, the shutter does not close after the completion of the illumination of the flash generator because of the under-exposure, continuing an automatic exposure controlling operation.
This will be more specifically described with reference to FIG. 1 which shows the integrated voltage of an arithmetic unit contained in the illumination control circuit of the auto strobo unit. In this Figure, the abscissa represents elapsed time T while the ordinate represents the integrated voltage V. When a photographing operation is initiated, a flash illumination is provided by the auto strobo unit at time t.sub.1. Assuming that the flash illumination reaches a given decision level V.sub.1 at time t.sub.2, an illumination stop signal is then produced to interrupt the illumination of the flash discharge tube. However, in the event of under-exposure, the flash discharge type of the auto strobo unit will be fully energized for its full period and terminate at time t.sub.3, as shown in FIG. 2. Still the integrated voltage V which varies in response to the amount of reflected light from an object being photographed fails to reach the decision level V.sub.1. As a consequence, the automatic exposure control operation continues its timing function, maintaining the shutter open. For a daytime synchronized photographing operation, natural light alone will be thereafter incident on the photometeric element to reach the decision level after an increased time length or at time t.sub.4, whereupon the shutter will be closed. Under this condition, the low illuminance of the environment is combined with the fact that the shutter is left open for a prolonged period of time after the termination of illumination from the flash discharge tube which depends on the value of diaphragm aperture, causing an unusual experience to the user. In addition, where a picture is taken while the camera is hand-held, a movement of the hands may cause doubled images. Therefore, it will be seen that the immediate closure of the shutter under these situations will result in a better photograph even though it may be exposed.